Single-ended-to-differential (or single-ended-to-balanced) signal converting circuits (baluns) have been widely employed in many radio frequency (RF), microwave and millimeter frequency applications. There have been many approaches and topologies proposed in previous works on the designs of baluns to meet various application demands. The Marchand balun, N. Marchand, “Transmission line conversion Transformers”, Electronics, vol. 17, pp. 142-145, 1944, has become one of the most popular balun topologies to provide low-loss and wide-band differential signals. An alternative topology is described in U.S. Pat. No. 6,292,070; and is often referred to as a back-wave balun. Both topologies can be realized using either distributed elements or lumped elements. And in both balun approaches, the balun comprises a first and second pair of coupled transmission line sections for distributed topology or pair of coupled transformer sections for lumped-element topology. The distributed topologies usually offer better bandwidth performance than their corresponding lumped-element solutions but at the cost of large circuit area, which corresponds to higher manufacturing cost. There have been several publications: Gavela, “A small size LTCC balun for wireless applications”, Proceedings of the European Microwave Conference 2004, pp 373-376;˜U.S. Pat. No. 6,819,199, on the size reduction using lumped-element versions for the above two balun topologies.
Many forms of Baluns are known in the art. See: Gavela, “A small size LTCC balun for wireless applications”, Proceedings of the European Microwave Conference 2004, pp 373-376; U.S. Pat. No. 6,819,199; Lin, “An Ultra-broadband Doubly Balanced Monolithic Ring Mixers for Ku- to Ka-band Applications”, IEEE Microwave and wireless components letters, Vol. 17, No. 10, October, 2007; Trifimovic, “Review of Printed Marchand and Double Y Baluns: Characteristics and Application”, EEE Transactions on Microwave Theory and Techniques, Vol. 42, No. 8, August, 1994;: Chen, “Novel Broadband Planar Balun Using Multiple Coupled Lines”, Microwave Symposium Digest, 2006, IEEE MTT-S International, pp. 1571-1574, as well as U.S. Pat. No. 6,683,510 B1 to Padilla, U.S. Pat. No. 7,250,828 B2 to Erb, U.S. Pat. No. 7,068,122 B2 to Weng, U.S. Pat. No. 6,275,689 B1 to Gill and U.S. Pat. No. 5,061,910 to Bouny. All these references are incorporated by reference herein.
Marchand balun's differential output branches are connected to ground via the second pair of the coupled sections while the back-wave balun's differential outputs are not grounded at the second pair of the coupled section. Therefore, when DC groundings of the differential ports are needed, the Marchand balun approach is preferred, and when non-zero DC biasing is needed for the differential output port, the back-wave balun approach is preferred. In addition, because the fabrication limitations and parasitic effects limit their bandwidth performance, both balun topologies have their own optimum operation frequency bands. Choosing between Marchand and back-wave baluns based on trade-off in DC biasing and bandwidth performance is often made for each specific application and available fabrication process requirements. In addition, the distributed strip-line baluns with tight broadside coupling are often used to improve bandwidth. But those strip-line baluns require multiple metal layers with rigorously controlled three-dimension profiles, which impose greater fabrication difficulties and higher cost for most planar and semiconductor integrate circuit fabrication processes. Single ended-to-balanced circuits (baluns) are bi-directional in concept, i.e., the input can be single ended and be converted to a differential or balanced output or the input can be balanced or differential and the output single-ended.